Dal form fai una SELECT al DB.
Se l'utente esiste:

$sql = "SELECT * FROM tabella where userid = '".$userid."' and password = '".$password."'" ;
$result = mysql_db_query($database,$sql,$db) or die("Errore ".mysql_error());

$num_rows = mysql_num_rows($result) ;
if ( $num_rows != 0 ) {
$row = mysql_fetch_array( $result );
session_register( "loggato" );
$loggato = true;
session_register( "userid" );
$userid = $row["userid"];
}else{
//non trovato
header ("Location: login.php");
}

Altra pagina:
<?
session_start();
Ciao $userid

etc.....
?>